484 SYNOPSIS 



PAGE 



d. The higher plants and animals begin life as a single cell, the 



ovum 68 



II. BIOGENESIS. 



a. Definition of Liojenesis and abiogenesis : brief history of 



the controversy , 95 



b. Crucial experiment with putrescible infusions : sterilisation : 



germ-filters : occurrence of abiogenesis disproved under 

 known existing conditions 98 



III. HOMOGENESIS. 



Definition of homogenesis and heterogenesis ; truth of the 



former firmly established 102 



IV. ORIGIN OF SPECIES. 



a. Meaning of the term Species : the question illustrated by a 



consideration of certain species of Zoothamnium . . . . 137 



b. Definition of Creation and Evolution : hypothetical histories 



of Zoothamnium in accordance with the two theories . . 141 



c. The principles of Classification : natural and artificial 



classifications 141 



d. The connection between ontogeny and phylogeny 146 



V. PLANTS AND ANIMALS. 



a. Attempt to define the words plant and animal, and to place 



the previously considered types in one or other king- 

 dom 176 



b. Significance of "third kingdom," Protista 182 



VI. SPERMATOGENESIS AND OOGENESIS. 



Origin of sperms and ova from primitive sex-cells : differences 



in structure and development of the sexual elements . . 253 



VII. MATURITION AND IMPREGNATION. 



a. Formation of first and second polar cells and of female 



pronucleus 257 



b Entrance of sperm and formation of male pronucleus . . . 260 

 >:. Conjugation of pronuclei 260 



